Lithium battery cathode

ABSTRACT

A novel lithium battery cathode, a lithium ion battery using the same and processes and preparation thereof are disclosed. The battery cathode is formed by force spinning. Fiber spinning allows for the formation of core-shell materials using material chemistries that would be incompatible with prior spinning techniques. A fiber spinning apparatus for forming a coated fiber and a method of forming a coated fiber are also disclosed.

STATEMENT OF GOVERNMENT INTEREST

The United States Government has rights in this invention pursuant toContract No. DE-AC04-94AL85000 between the United States Department ofEnergy and Sandia Corporation for the operation of the Sandia NationalLaboratories, and between the University of Texas-Pan American and theNational Science Foundation under Grant No. NSF DMR0934157.

FIELD

The present disclosure is generally directed to energy storage, and ismore particularly directed to a coated, fibrous cathode material for alithium battery and a method for making this material.

BACKGROUND

A lithium ion battery is a chemical power source commonly referred to asa secondary battery. Lithium batteries include a cathode formed from acompound able to reversibly intercalate and de-intercalate lithium ion,and an anode formed from other compound(s) able to reversiblyintercalate and de-intercalate lithium ion. When the battery is charged,lithium ion is de-intercalated from its cathode and intercalated intoits anode. The reverse takes place when the battery is discharged. Alithium ion battery basically comprises an electrode core and anonaqueous electrolyte, both sealed in the battery case. The electrodecore comprises a battery electrode comprising an anode, a cathode and amembrane used to separate the anode from the cathode. The cathodecomprises a current collector and a cathode material coated on and/orfilled in the current collector. The cathode material comprises acathode active substance, a conductive additive and an adhesive.

Energy storage technology is recognized as a critical need formaintaining the quality of life in developed and developing nations.Demand for lithium-ion batteries (LIB) are rapidly increasing due to itsapplication in wide range of devices such as cell phones, laptopcomputers, and camcorders, as well critical health applications such asa cardiac pacemaker where an energy source is required to assist regularheart-rhythm. Power needs are also critical for advanced militaryequipment, off-peak power for intermittent power generation sourcesincluding solar cells and wind generators, in electric vehicles, orspace applications. The power source for these devices should possesshigh specific capacity (Ah/g) and density (Whig or Wh/l) that will makethe batteries lighter and smaller, respectively.

The interfaces in cathode materials dictate numerous properties ofelectrode storage materials ranging from capacity loss, Li⁺ transportactivation energy cost, as well as electrolyte stability and degradationproducts. An optimized interfacial layer that creates a stable transportinterface is critical to the effective application of energy storagematerials. The formation of controlled interfacial layers has not beensystematically examined due to the typical architecture of the cathodeelement in battery components, which consist of carbon black, binder andactive material powders. Active materials research is directed to bulkmodifications through doping during synthesis processes, and theformation of controlled interfaces have not been addressed in detail.There are several cathode materials for lithium ion batteries known tohave performance limits or progressive degradation due to reaction ofthe cathode material with the electrolyte to form a solid electrolyteinterphase-like layer (SEI-like). Surface protection of nanoparticlecathode materials is used to improve overall battery performance, wherethe surface coatings are applied in a second synthesis step to the coreparticle synthesis. These coatings can be either organic (i.e. carbon)or inorganic ceramics (i.e. metal oxides).

In 1991, Sony commercialized the lithium rocking chair battery, whichnow is commonly known as lithium ion batteries. The pioneer introductionof LiCoO₂ cathode in rechargeable (secondary) cell had a very highvoltage and energy density during that period. But LiCoO₂ batteries areexpensive and are toxic. LiMn₂O₄ spinel is an excellent cathode materialfor the replacement of LiCoO₂ because it is cheap, environmentallybenign, exhibits good thermal behavior and a fairly high dischargevoltage. However, LiMn₂O₄ has several disadvantages such as capacityreduction during charge-discharge (CD) cycle due to phase transition,and structural instability due to Mn dissolution in the electrolyte byacid attack. Hydrofluoric acid (HF) generated by the fluorinatedelectrolyte salts is one such example for Mn dissolution and is given asbelow,2LiMn₂O₄→3λ-MnO₂+MnO+Li₂O  (1)

However, manganese (through MnO) and lithium (through Li₂O) dissolvesinto the electrolyte, which results in capacity fade at elevatedtemperature (40-50° C.), reducing battery performance.

The need remains, therefore, for a lithium battery having improvedbattery performance, and in particular, for a lithium battery cathodehaving high specific energy, good electrochemical cycling stability,electrode capacity, cycle life, good electronic conductivity, highlithium diffusivity, and chemical compatibility to the electrolyte, aswell as low cost, safety, and a benign environment. The need alsoremains a need for a method of forming a lithium battery having improvedbattery performance, and in particular, for forming a lithium batterycathode having high specific energy, good electrochemical cyclingstability, electrode capacity, cycle life, good electronic conductivity,high lithium diffusivity, and chemical compatibility to the electrolyte,as well as low cost, safety, and a benign environment

SUMMARY OF THE DISCLOSURE

The present invention is directed to novel lithium battery cathode, alithium ion battery using the same and processes and preparationthereof. The battery cathode fibers are formed by a high speed, fiberspinning method that allows for the formation of core-shell materials atrapid production rates. The fiber spinning forms the coated fibers fromprecursor polymer solutions formed by centrifugal forces and ambientatmospheric inertial drag to form diameters from the micron tonanoscale, and lengths ranging from 10-1000 mm or as continuous fibers.In an embodiment, the coated fibers may be formed by a Forcespinning™method and apparatus, which is a high rotational speed, fiber spinningtechnique. Forcespinning™ is a registered trademark of FiberioTechnology Corporation, a Delaware Corporation.

In an embodiment, the present invention is directed to coating a cathodematerial to improve the cathode performance in LIB. In an embodiment,LiMn₂O₄ spinel cathode material may be coated with ZrO₂ to serve as aprotective barrier layer against electrochemical cycling degradation.The surface of the coated cathode spinel suppresses the Mn dissolutionby preventing dissolution of Mn ions. Also, the coating reduces thecubic-tetragonal phase transition and improved the stability of thespinel structure.

According to an embodiment, a lithium battery is disclosed that includesa cathode, an anode, an electrolyte between the anode and cathode, and aseparator disposed between the anode and cathode. The cathode includes aplurality of fibers having a core surrounded by a shell, wherein thefibers are formed by fiber spinning.

According to another embodiment of the invention, a lithium batterycathode is disclosed that includes a plurality of fibers having a coresurrounded by a shell, wherein the fibers are formed by fiber spinning.

According to another embodiment of the present invention, a method ofmaking a lithium battery cathode is disclosed that includes forming aplurality of precursor fibers by fiber spinning a fiber having a coreand shell, and calcinating the plurality of precursor fibers to form thecathode.

According to another embodiment of the present invention, an apparatusfor fiber spinning coated fibers is disclosed that includes a spinnerethaving an inner needle surrounded by an outer needle for forming spunfibers, a material supply system providing a core precursor material tothe inner needle and a shell precursor material to the outer needle, anda collection system for collecting the spun fibers from the spinneret.The spinneret rotates at a rotational speed greater than zero and up toabout 15,000 rpm. In another embodiment, the spinneret rotates at arotational speed greater than zero and up to about 10,000 rpm.

According to another embodiment of the present invention, a method offiber spinning a coated fiber is disclosed that includes providing acore precursor material to a spinneret, providing a shell precursormaterial to the spinneret, rotating the spinneret while spinning thecore and shell precursor materials to form the coated fiber, andcollecting the coated fiber.

One advantage of the present disclosure is to provide a lithium batterythat prevents cathode degradation and capacity cycling loss over thelifetime of the battery.

Another advantage of the present disclosure is to provide cathode thatprovides high specific energy, good electrochemical cycling stability,good electronic conductivity, high lithium diffusivity, and chemicalcompatibility to the electrolyte, as well as low cost, safety, and abenign environment.

Other features and advantages of the present disclosure will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the features and advantages of the present invention can beunderstood in detail, a more particular description of the invention maybe had by reference to the embodiments thereof that are illustrated inthe appended drawings. These drawings are used to illustrate onlytypical embodiments of this invention, and are not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments. The figures are not necessarily to scale andcertain features and certain views of the figures may be shownexaggerated in scale in schematic in the interest of clarity andconciseness.

FIG. 1 illustrates an embodiment of a lithium ion battery according tothe present invention.

FIG. 2 illustrates an embodiment of a cathode according to the presentinvention.

FIG. 3 illustrates an embodiment of a cross section of a cathode fiberaccording to the present invention.

FIG. 4 illustrates an embodiment of a fiber spinning apparatus accordingto the present invention.

FIG. 5 illustrates an embodiment of a spinneret according to the presentinvention.

FIG. 6 illustrates a simplified schematic of an embodiment of acollection system according to the present invention

FIG. 7 illustrates a simplified schematic of another embodiment of acollection system according to the present invention

FIG. 8 shows an embodiment of a flow chart of a method of forming aglass composite waste form according to the present invention.

FIG. 9 shows a comparison of electrochemical cycling stability oflithium cells with (a) uncoated LiMn₂O₄ and (b) ZrO₂-coated LiMn₂O₄electrode [3].

Wherever possible, the same reference numbers will be used throughoutthe drawings to represent the same parts.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete and will fully conveythe scope of the invention to those skilled in the art.

FIG. 1 illustrates a schematic of a general Li-battery 10 according tothe present invention. As can be seen in FIG. 1, the battery includes acathode 110, an anode 120, a separator 130, and an electrolyte 140. Thecathode 110 is electrically connected via a first current collector 150,and the anode 120 is connected by a second current collector 160 to adischarge device 170. In an embodiment, the first current collector 150may be formed of a suitably non-reactive, conducting material, such as,but not limited to aluminum. In an embodiment, the second currentcollector 160 may be formed of a suitably non-reactive, conductingmaterial, such as, but not limited to copper. The parts of the generalLi-battery 10, other than the novel structure of the cathode 110, arewell understood in the art and are not further described herein.

FIG. 2 illustrates an embodiment of the cathode 110 according to thepresent invention. As can be seen in FIG. 2, the cathode 110 is formedof a fiber mat 112. The fiber mat 112 is formed by a plurality ofconductive fibers 114. The fibers 114 have a length greater than about 5microns. In another embodiment, the fibers 114 may have a length betweenabout 5 microns and 7 feet. In another embodiment, the fibers 114 have alength between about 1 mm and about 1000 mm. In another embodiment, thefibers 114 have a length of about 10 mm to about 100 mm. In anotherembodiment, the cathode 110 may be formed of a continuous fiber.

The fibers 114 range in diameter from about 200 nm to about 5 microns.In another embodiment, the fibers 114 range in diameter from about 500nm to about 3 microns. In another embodiment, the fibers 114 range indiameter from about 200 nm to about 500 nm.

FIG. 3 shows a cross section of a fiber 114 of FIG. 2. As can be seen inFIG. 3, the fiber 114 includes a core 310 and a shell 320. The core 310has an average diameter between about 150 nm and less than about 5microns. In another embodiment, the core 310 has an average diameterbetween about 150 nm and about 4.995 microns. In another embodiment, thecore has an average diameter between about 300 nm and about 1 micron. Inan embodiment, the core 310 has an average diameter of about 475 nm.

The shell 310 has a thickness between about 5 nm and about 400 nm. Inanother embodiment, the shell 310 has a thickness between about 5 nm andabout 20 nm. In an embodiment, the shell 310 has a thickness betweenabout 5 nm and about 10 nm.

The core 310 is formed of a crystalline metal oxide material capable ofintercalating and de-intercalating lithium ions In addition, the core310 may have material gradients of one or more of the shell materials,in which the shell chemistry or composition varies without a distinctphase boundary or as a mixture of discrete particles forming a gradualtransition in materials. The crystalline oxide material may be amanganese or cobalt based compound. In an embodiment, the manganesebased oxide may be LiMn₂O₄. In another embodiment, the LiMn₂O₄ oxide mayfurther include one or more elements selected from the group includingnickel, chromium, gallium, iron. In an embodiment, the LiMn₂O₄ oxide maybe LiMn_(1.5)Ni_(0.5)O₄ or other mixtures of suitable cations.

In an embodiment, the cobalt based oxide may be LiCoO₂. In anotherembodiment, the LiCoO₂ oxide may further include one or more elementsselected from the group including manganese and nickel. In anotherembodiment, the LiCoO₂ oxide may be LiNi_(0.33)Mn_(0.33)Co_(0.33)O₂ orother mixtures of suitable cations.

The crystalline metal oxide core has a grain size between about 10 nmand about 100 nm. In another embodiment, the grain size may be betweenabout 20 nm and about 40 nm.

The shell 320 is formed of shell material that is conformal, continuous,dense, lithium transporting, chemically stable and non-reactive to theelectrolyte. The shell material may include one or more layers formed ofa non-reactive ceramic oxide, an electrochemically active metal oxide,and a conductive metal oxide. In addition, the shell 320 may havematerial gradients of one or more of the shell materials, in which theshell chemistry or composition varies without a distinct phase boundaryor as a mixture of discrete particles forming a gradual transition inmaterials. Also, the shell 320 may include carbon based layers.

The non-reactive ceramic oxide serves to prevent electrochemicalbreakdown of the electrolyte phase, or degradation of the cathode bymechanical stress or electrochemical dissolution or degradation of thecathode by phase transition. In an embodiment, the non-reactive ceramicoxide may be selected from the group including ZrO₂, Al₂O₃, TiO₂, MgO,SiO₂, Cr₂O₃ and mixtures thereof In an embodiment, the non-reactiveceramic oxide may include lithium variants in which one or more of theoxides containing a single cation [Mx+] become compounds of Li and thecation as an oxide phase (e.g. TiO₂ becomes LiTi₂O₅).

The electrochemically active metal oxide serves as a cathode in thebattery design. In an embodiment, the electrochemically active metaloxide may be selected from the group discussed above in describing thecore, and may also include nickel oxides, cobalt oxides, and dopedversions thereof.

The conductive metal oxide serves as a barrier layer having theadditional property of significant electronic conduction allowing forcharge transport between fibers in the cathode. In an embodiment, theconductive metal oxide may be selected from the group includingruthenium oxide, indium tin oxide, and doped variants thereof. In anembodiment, the conductive metal oxide may include lithium variants.

The carbon based layers may be selected from the group includinggraphitic carbon, grapheme, oxidized graphite, or crosslinked polymershell encapsulants, such as, but not limited to a polyacrylonitrile.

FIG. 4 illustrates an embodiment of a fiber spinning apparatus 400 usedto form precursor fibers 410 according to the present invention. As canbe seen in FIG. 4, the fiber spinning apparatus 400 includes a spinneret420, a collection system 430, a drive system 440, and a precursormaterial supply system (not shown). The drive system 440 includes amotor 442 and a drive shaft 444 that is rotationally driven by the motor442. The driveshaft 444 mechanically rotates the spinneret 420. In thisexemplary embodiment, the driveshaft 444 includes a precursor materialsupply system that provides the precursor materials through pipingwithin the driveshaft 444 to the spinneret 420 to be spun into fibers.In another embodiment, the precursor material supply system may not bewithin the driveshaft, but may be provided to the spinneret by amaterial feed system in material connectivity to the spinneret, such asfor example, from a material feed system positioned above the spinneret.

The drive shaft 444 rotates the spinneret between 0 and 20,000 rpm. Thedriveshaft 444 rotates the spinneret between 0 and 15,000 rpm. Inanother embodiment, the driveshaft 444 rotates the spinneret betweenabout 3000 to about 10,000 rpm. In another embodiment, the spinneretrotates between about 5,000 rpm and 8,000 rpm.

FIG. 5 illustrates a simplified schematic of an embodiment of thespinneret 420 according to the present invention. In this simplifiedschematic, the material supply connections, at least, are not shown forclarity. As can be seen in FIG. 5, the spinneret 420 includes aspinneret body 525 and opposing needles or nozzles 530. The spinneretbody 525 includes a core fluid distribution portion 510 and a shellfluid distribution portion 520. In this exemplary embodiment, thespinneret body 525 has a generally cylindrical geometry. In anotherembodiment, the spinneret body may be square, pentagonal (in the shapeof a star such that each point of the star is pointing radially),hexagonal, octagonal or other geometric shape such that the opposingsides are perpendicular to the radial direction of rotation.

The opposing nozzles 530 include a core discharge nozzle 512 in fluidcommunication with the core fluid distribution portion 510 and having afirst opening 513 having a first inside diameter D1 of 100 to 200microns, and a shell discharge nozzle 522 in fluid communication withthe shell distribution portion 520 and having a second opening 523having a second inside diameter D2 of about 300 to about 500 microns. Inanother embodiment, the nozzles may not be concentric and/or circular.For example, the nozzles may be oval, square, rectangular, hexagonal, orother geometric shape.

In this exemplary embodiment, the spinneret 420 includes one set ofopposing nozzles 530. In another embodiment, the spinneret 420 mayinclude one or more nozzles. In another embodiment, the spinneret 420may include two or more nozzles. In another embodiment, the openings ofthe two or more nozzles may be offset so as to form a non-uniform shellsurrounding the core.

In another embodiment, the nozzles 530 may be deleted, and the core andshell discharge openings may be nested on a surface of the spinneretbody 525. As used herein, the term nested means that the inner openingis surrounded by the outer opening. For example, the openings may benested on the tips of a star or pentagonal shaped spinneret, or on theradial surfaces of a cube or other geometric shape.

The nozzles 530 extrude the core and shell fluids in a core and shellgeometry. In other words, the shell material is extruded around theshell material and the spinneret is rotated. The nozzles 530 have anexit opening 532 of between 100 to 500 microns. In an embodiment, theouter needle is 21 gauge and inner needle is 30 gauge.

In another embodiment, the nozzles 530 may include two or moreconcentric nozzles. In such a manner, a precursor fiber 410 (FIG. 4) maybe spun that includes two or more core materials and/or two or moreshell materials. In an embodiment, the nozzles need not be concentricand may be offset to create a conformal coating from the spinningprocess.

FIG. 6 shows an embodiment of a collection system 430 according to thepresent invention. As can be seen in FIG. 6, the collection system 430includes a plurality of pins 610 and a base 620 to which the pins 610are attached. The collection system 430 collects or gathers fibers asthey are discharged from the rotating spinneret 420. In this embodiment,the collection system 430 includes 10 pins. In another embodiment, thecollection system 430 may include (Inventors, what is the minimum numberof pins). In another embodiment, the collection system may include 10 ormore pins 610. In this exemplary embodiment, the pins 610 are straight.In another embodiment, the pins 610 may be curved.

In this exemplary embodiment, the pins 610 are removable or detachablefrom the base 620. In such a manner, the discharged fibers may becollected or gathered from the collection system by gathering thedischarged fibers onto one or more pins 610 by sweeping the one or morepins through the discharged fibers (see FIG. 7). In another embodiment,the pins 610 may not be detachable from the base 620. In thisembodiment, the discharged fibers collected on the pins 610 arecollected from the pins 610 by gathering with a gathering tool such as awand, pin, scoop, or other similar gathering instrument. In thisexemplary embodiment, the collection system 430 is stationary while thespinneret 420 rotates. In another embodiment, the collection system 430may rotate. In another embodiment the collection system 430 may rotateat the same rate as the spinneret 420. In another embodiment, thecollection system 430 may rotate at a different rate than the spinneret420.

FIG. 7 shows another embodiment of a collection system 710 according tothe present invention. As can be seen in FIG. 7, a drum 720 has replacedthe pins 620 (FIG. 6) for the gathering or collection of the spunfibers. The drum 720 includes and inside surface 722 upon which fibers(not shown) gather or collect as they are discharged from the rotatingspinneret 420. In an embodiment, the drum 720 may be removable ordetachable from the base 620. The discharged fibers may be collected orgathered from the drum 720 by gathering the discharged fibers onto oneor more collection or gathering tools (not shown). The gathering toolmay be a pin, wand, scoop, or other similar gathering instrument. Inthis exemplary embodiment, the drum 720 is stationary while thespinneret 420 rotates. In another embodiment, the drum 720 may rotate.In another embodiment the drum 720 may rotate at the same rate as thespinneret 420. In another embodiment, the drum 720 may rotate at adifferent rate than the spinneret 420. In another embodiment, thecollection system 430 may include pins, drums, wires, open mesh, orother collection surfaces for gathering or collecting the spun fibers.

FIG. 8 shows an embodiment of a method of forming a plurality ofprecursor fibers having a core and shell structure by fiber spinningaccording to the present invention. In an embodiment, the coated fibersmay be formed by a Forcespinning™ method and apparatus, which is a highrotational speed, fiber spinning technique. Forcespinning™ is aregistered trademark of Fiberio Technology Corporation, a DelawareCorporation.

As can be seen in FIG. 8, a first step 810 includes forming precursorsolutions. The core precursor solution is formed of a molecularprecursor or precursors that can be spun and calcinated to form thedesired core material.

The core precursor solution includes a solvent, a polymer materialdissolved in the solvent, and a molecular precursor to the corematerial. The polymer may be selected from the group of polyvinylalcohol (PVA), polyethylene oxide (polyethylene glycol (PEG) or a blendof PVA and polyvinyl pyrrolidone (PVP). In an embodiment, the weightpercent (wt % of PVP in the blend of PVA and PVP may be between 1 to 25weight %, with the operational range dependant on polymer molecularweight, solvent drying rate, plasticizer effect, and operatingrotational speed. As an example, 10 weight % of PVP polymer at 1.3 Mmolecular weight in ethanol is capable of forming fibers between3000-8000 rpm.

The molecular precursor may be a dissolved component, a particlecomponent, or a combination of both. In an embodiment, the dissolvedcomponent may be one or more components selected from the groupincluding dissolved metal salts, sol-gels, chemically stabilizedprecursor materials and chelated molecules. In an embodiment, thedissolved metal salt may be a metal chloride, nitride or nitrate. Thiscan include LiCl, LiNO₃, LiCO₃ or Mn(NO₃)₄, Manganese acetate Cobaltacetate or cobalt nitrate. In an embodiment, the metal oxide precursormay be an acetate, oxalate, metal alkyl compounds.

In an embodiment, the chelated molecules may include as a chelatingagent acetylacetone, citrate ions, citric acid, tartaric acid, malicacid, lactic acid, ethylenediamine, ethylenediamine tetracetic acid andother chelating agents effective to prevent hydrolysis or precipitationof metal cation species during preparation and the spinning process Thenanoparticles may be single component oxides such as Li₂O, MnO₂Mn₂O₃,hydroxides of cations (e.g. Mn(OH)₄, Co(OH)₂), or nanoparticles of thedesired cathode material (LiMn₂O₄ spinel, LiCoO₂) The nanoparticles maybe selected from the group including Li, Mn, Co, Ni, Ga, and Fe.

The shell precursor solution includes a solvent, a polymer materialdissolved in the solvent, and a molecular precursor to the corematerial. The polymer may be selected from the group of polyvinylalcohol (PVA), polyethylene oxide (polyethylene glycol (PEG) or a blendof PVA and polyvinyl pyrrolidone (PVP). In an embodiment, the weightpercent (wt % of PVP in the blend of PVA and PVP may be between about 1to about 25 weight %, with the operational range dependant on polymermolecular weight, solvent drying rate, plasticizer effect, and operatingrotational speed. As an example, 10 weight % of PVP polymer at 1.3 Mmolecular weight in ethanol is capable of forming fibers between3000-8000 rpm.

In an embodiment, the dissolved component may be one or more componentsselected from the group including dissolved metal salts, sol-gels,chemically stabilized precursor materials and chelated molecules. In anembodiment, the dissolved metal salt may be a metal chloride, nitride ornitrate, or a metal alkyl compound or a metal alkyl compound reactedwith a chelating agent. Examples include TiCl₄, Titanium isopropoxide,aluminum tri-sec-butoxide, and zirconium butoxide. In an embodiment, themetal oxide precursor may be acetate, oxalate, metal alkyl compounds.

In an embodiment, the chelated molecules may include as a chelatingagent acetylacetone, citrate ions, citric acid, tartaric acid, malicacid, lactic acid, ethylene diamine, ethylenediamine tetracetic acid andother chelating agents effective to prevent hydrolysis or precipitationof the component materials during preparation or processing to formfiber materials.

The nanoparticles may be hydroxides or oxides of the desired shellmaterial (e.g. TiO2, ZrO2, SiO2, MgO, Al2O3), or direct oxides of thedesired shell material, as well as combinations or disparate materials.The nanoparticles may be selected form the group including TiO₂, ZrO₂,SiO₂, MgO, Al₂O₃, RuO₂, indium tin oxide.

Additionally, the core and shell precursor solutions may further includerheology agents, plasticizing agents to improve control over fiberdimension, surfactants to stabilize core and shell precursor formation,and other solution modification agents may be used. In an embodiment,the rheology agents may be higher boiling point solvents such as, butnot limited to ethylene glycol, polyethylene glycol, polyethylene oxide,dimethyl formamide, and N-methyl formamide.

Referring again to FIG. 8, a second step 820 including spinning theprecursor solutions by a fiber spinning apparatus to form a core/shellprecursor fiber. The precursor fibers are spun at a rotational rateselected based on the precursor compositions so as to form stand alone,self-supporting, precursor fibers. In an embodiment, the core and shellprecursor materials and fiber spinning apparatus parameters arecritically selected so the precursor shell material completely surroundsthe core precursor material.

Referring again the FIG. 8, a third step 830 includes collecting thespun precursor fibers. The precursor fibers are collected from the fiberspinning apparatus, and shaped into a desired fiber mat shape. Theprecursor fibers may optionally be dried prior to or during shaping. Theprecursor fibers may be formed into open or loose structures, layers,pressed compacts, felts, yarns or coatings to other materials or currentcollectors.

A fourth step 840 includes calcinating the shaped precursor fibers toform the crystalline metal oxide fibers. Calcinating is performed at atemperature between about 500° C. and about 900° C. In an embodiment,the calcinating temperature is between about 650° C. and about 700° C.being sufficient for highly crystalline performing cathode materials.The calcination may be performed in air or other atmospheres, includingoxidizing and reducing atmospheres, to control grain structure and/ormechanical integrity. In an embodiment, the precursor fibers may becalcinated in air at 650° C. for 2 hours.

According to an optional fifth step 850, the calcinated fibers areshaped and or aligned into a matt, yarn, disk or other applicable shape.The shaping may include compressing the fibers into a compressed shape.

According to a sixth step 860, the crystalline metal oxide fibers areintegrated into a battery assembly. The fibers are densely packed onto acurrent collecting electrode, and matched with a separator and anodeassembly.

Coated fibers formed by the above described method have improvedstability compared to uncoated fibers. The coating suppresses the Mndissolution by scavenging unwanted HF from the electrolyte. Also, thecoating helped to reduce the cubic-tetragonal phase transition andimproved the stability of the spinel structure. FIG. 9 shows theimproved stability of the ZrO₂ coated LiMn₂O₄ spinel cathode particlesat elevated temperature

Example 1

A core precursor solution was prepared using lithium chloride andmanganese acetate in a molar ratio of 1.2:2, which then dissolved in apolymer solution of mixture of polyvinyl alcohol (PVA; 10 wt % in DIwater) and polyvinylpyrrolidone (PVP; 20 wt % in DI water) A shellprecursor was prepared by mixing zirconium acetate and PVP at 20 weight% in 2-propanol. The precursors were fed into the spinneret and spunusing the fiber spinning technique described above. The initialexperiments were carried out using outer needle (shell material) of 21gauge and inner needle (core material) of 30 gauge. At rotational speedof 4000 rpm uniform and even coaxial ultrathin fibers were prepared. Thefibers were then fired at 700° C. for 5 hrs. After the calcination, thefibers retain their morphology and high surface area. The fiber diameterwas 2 to 3 microns in diameter.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the appended claims. It is intendedthat the scope of the invention be defined by the claims appendedhereto. The entire disclosures of all references, applications, patentsand publications cited above are hereby incorporated by reference.

In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the disclosure withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the disclosure not be limited to the particular embodimentdisclosed as the best mode contemplated for carrying out thisdisclosure, but that the disclosure will include all embodiments fallingwithin the scope of the appended claims.

What is claimed is:
 1. A lithium battery, comprising: a cathode; ananode; an electrolyte between the anode and cathode; and a separatordisposed between the anode and cathode; wherein the cathode comprises: aplurality of fibers comprising a core surrounded by a ZrO₂ shellcompletely surrounding the core; wherein the fibers are formed by fiberspinning.
 2. The battery of claim 1, wherein the core comprises acrystalline metal oxide material.
 3. The battery of claim 1, wherein thecrystalline metal oxide is LiMn₂O₄ or LiCoO₂.
 4. The battery of claim 1,wherein the core is formed of oxide particles having a particle sizebetween 10 to about 100 nm.